Conjugated diene polymers are most often produced by solution polymerization, wherein conjugated diene monomers are polymerized in an inert solvent or diluent. The solvent serves to solubilize the reactants and product, to act as a carrier for the reactants and product, to aid in the transfer of the heat of polymerization, and to help in moderating the polymerization rate. The solvent also allows easier stirring and transferring of the polymerization mixture (also called cement), since the viscosity of the cement is decreased by the presence of the solvent. Thermal control is much easier in solution polymerization than in bulk polymerization. Nevertheless, the presence of solvent presents a number of difficulties. The solvent must be separated from the rubber and then recycled for reuse or otherwise disposed of. The cost of recovering and recycling the solvent adds greatly to the cost of the rubber being produced, and there is always the risk that the recycled solvent after purification may still retain some impurities that will poison the polymerization catalyst. In addition, some solvents such as aromatic hydrocarbons can raise environmental concerns. Further, the purity of the polymer product may be affected if there are difficulties in removing the solvent.
In bulk polymerization (also called mass polymerization), the monomer is polymerized in the absence or substantial absence of any solvent, and, in effect, the monomer itself acts as a diluent. Since bulk polymerization is essentially solventless, there is less contamination risk, and the product separation is simplified. Bulk polymerization offers a number of economic advantages including lower capital cost for new plant capacity, lower energy cost to operate, and fewer people to operate. The solventless feature also provides environmental advantages, with emissions and waste water pollution being reduced. Nonetheless, bulk polymerization requires very careful temperature control to avoid a runaway reaction, and there is also the need for strong and elaborate stirring equipment since the viscosity of the polymerization system can become very high.
In bulk polymerization, lanthanide-based catalyst systems that comprise a lanthanide compound, an aluminoxane, and a halogen source are known to be useful for producing conjugated diene polymers having high cis-1,4-linkage contents. The resulting cis-1,4-polybutadienes typically have a cis-1,4-linkage of more than 98.5% and 1,2-linkage lower than 0.3% with molecular weight distributions that vary, but are typically below 2. It is known that cis-1,4-polybutadienes having high cis contents, low vinyl contents, and narrow molecular weight distributions give a greater ability to undergo strain-induced crystallization and lower hysteresis and thus, give superior physical properties such as higher tensile strength and higher abrasion resistance.
Syndiotactic 1,2-polybutadiene is a crystalline thermoplastic resin that has stereoregular structure in which the side-chain vinyl groups are located alternately on the opposite sides in relation to the polymeric main chain. Syndiotactic 1,2-polybutadiene exhibits the properties of both plastics and rubber and is blended into or co-cured with natural and synthetic rubbers. When small amounts of syndiotactic 1,2-polybutadiene is combined with cis-1,4-polybutadiene, the resulting polymer blends exhibit improved cold flow resistance, improved green strength, and improved wear characteristics.
Blends of syndiotactic 1,2-polybutadiene and cis-1,4-polybutadiene are often prepared by mixing the two polymers together at elevated temperatures. This can be problematic because the melting temperature of the syndiotactic 1,2-polybutadiene is often in excess of 180° C. which is above the thermal degradation temperature of the cis-1,4-polybutadiene. Alternatively, the syndiotactic 1,2-polybutadiene can be synthesized within the same vessel as the 1,4-polybutadiene, which thereby provides an even distribution of syndiotactic 1,2-polybutadiene within the cis-1,4-polybutadiene. Often, the different catalysts used to synthesize the two polymers are not compatible with each other and synthetic schemes have to overcome catalyst incompatibility. The in-situ synthesis of the polymer blends also occurs in solution, which has the disadvantages noted above.
Therefore, there exists a need to develop new methods for producing blends of cis-1,4-polybutadiene and syndiotactic 1,2-polybutadiene.